Powder metallurgy



3,924,127 Patented Mar. 6, 1962 3,024,127 PQER METALLURGY This invention relates to the art of powder metallurgy and more particularly to the application of molybdenum disulfide to machine elements which are to be subjected to further operations. This invention relates especially to the coining of powdered metal compacts, usually in sintered condition, whereby machine elements are produced of high density, closer dimensional tolerance, and finer surface finish than heretofore possible; and the invention also includes the resulting machine elements thus obtained, both in intermediate and finished stage, and also includes methods and compositions used in the application of molybdenum disulfide to such elements.

Dense, precision powdered metal compacts e.g. for machine elements, are desirably made by mixing base metal powder (e.g. ferrous met-a1 powder) either with or without (as required in the end product), a predetermined amount of powdered alloying material, and powdered dry lubricant. This mixture is then initially pressed in a die at high pressures, in excess of forty (40) tons per square inch. This initial pressing pressure should be as high as economically feasible in order to generate as high an initial density as possible from a practical standpoint. Following this operation the parts are sintered at a predetermined temperature and for a predetermined time desirably, in a reducing atmosphere, in order to permit grain growth or knitting between individual particles and to eliminate the work hardening effects of the pressing operations. This operation also causes the included dry lubricant to burn out. A repressing or coining operation is then performed on the part using pressures of usually eighty (80) tons per square inch or less. This latter operation imparts to the parts their final density, approximate dimensions, and surface finish. The parts so produced have properties analogous to low carbon steels made by conventional methods and exhibit dimensional tolerances that may be closely maintained throughout subsequent processing operations.

'The presence of a lubricant in the powder during the initial pressing operation minimizes the troubles at that time when the powders have not alloyed to create harder and stronger materials which are more resistant to the deformation required to compact them into the desired shape. After the compacts so formed are sintered, however, the lubricating media has been driven off and stronger, harder alloys have been formed. When these parts are then repressed as in coining, it is a requisite to have a satisfactory lubricant present in order to overcome friction between the parts and the die, to prevent galling or seizure between the parts and the die, to reduce the ejection pressure required to remove the parts from the dies after repressing, and to promote the formation of a very smooth surface on the part processed. A great variety of methods and materials have been utilized to provide a satisfactory lubricant for this last-named operation but most, if not all, of them have proven inadequate for one reason or another. This has been true particularly of wet lubricants or of lubricants applied to the parts by means of a liquid or solution. In such cases, the liquid media enter the pores of the compact and cannot be completely removed from these areas and thus inhibit the formation of high density parts and high surface finishes. Another disadvantage in using most of the commonly accepted lubricants is their inability to provide adequate lubrication under the high pressure and temperature conditons which are encountered in the repressing operation.

Among the objects of the present invention is included the utilization of molybdenum disulfide as a lubricant for machine elements particularly in the form of powder metal compacts which have been sintered, to supply the necessary lubricant in subsequent operations to which such elements may be subjected, particularly in coining operations.

Other objects include the coining or repressing of such machine elements which carry molybdenum disulfide as a lubricant.

Still further objects include methods and compositions for applying molybdenum disulfide to machine elements, particularly to powder metal compacts.

Still further objects include the elements carrying surface deposited molybdenum disulfide and the final coined articles produced therefrom.

Still further objects and advantages will appear from the more detailed description set forth below, it being understood that such more detailed descripton is given by way of illustration and explanation only, and not by way of limitation, since various changes therein may be made by those skilled in the art, without departing from the scope and spirit of the present invention.

In accordance with the present invention it has been found that molybdenum disulfide is a particularly valuable lubricant for utilization during coining of powder metal compacts provided that the molybdenum disulfide is applied to the surfaces only of the compacts as a substantially uniform but thin layer, sufiicient to serve as a lubricant during any such subsequent treatment, as for example, repressing or coining. It is of greatest importance that the lubricant should be surface deposited only. The compacts are of porous absorbent character so that any penetration of lubricant during mechanical treatment will result in loss of strength as by lowered density, and will also militate against obtaining the highest possible surface finish.

For these purposes the lubricant is applied in a completely dry condition and the method of application is such that a very thin, uniform film of the lubricant is applied to the surface only and in such a manner that there is no abrading or marring of the surfaces of the parts during the application. The molybdenum disulfide lubricant, itself, is far superior to any yet employed in that it provides greater lubricating qualities under high pressure and high temperature conditions thereby resulting in more etficient use of applied pressures, the development of superior surface finishes, the maintenance of closer dimensional tolerances, and the possibility of using higher rates of production. It has also been found that the thin film of molybdenum disulfide so formed is maintained after the repressing operation has been completed and as such film it has been found to impart, particularly to ferrous base components, an improved degree of corrosion resistance.

To accomplish the desired results, the molybdenum disulfide in dry finely divided condition is applied to the machine elements, such as powder metal compacts, by intimately contacting the elements with the dry lubricant in the presence of solid dry adsorbent material for the lubricant, or material which has a high aflinity for molybdenum disulfide, the material being substantially softer than the metal of the element, the intimate contact being continued until the element has received a surface coating of dry disulfide. Desirably, the adsorbent material has first been coated with the lubricant. In any event, the adsorbent material serves as a carrier for the dry lubricant and transfers the dry lubricant to the surface only of the machine element.

A simplified fiow diagram of the method, with the alternate method indicated in dotted lines, is as follows:

Tumble coat i 1 with 2 1 Tumble 1, 2 and 3 with at least 2 parts of 1+2 to 1 part of 3 Rc-press, re-coln or re-sinter 3 The molybdenum disulfide is used in finely divided dry condition, the particle size enabling surface adherence particularly when applied as herein set forth. Particle size should desirably be below about 10 microns down to 1 micron and smaller. Where the surfaces to be coated are not cut by small spaces as between gear teeth, the size of particle may be larger e.g. up to about 140 to 150 microns.

This may be herein accomplished by taking a quantity of a carrier or carrying agent such as ordinary small wire brads which have preferably been hardened, ground corn cobs, wood chips, leather particles, rubber particles, or particles of any of a great variety of similar adsorbent materials, softer than the metal of the machine element, and charging them with a fine particle size grade of molybdenum disulfide. This operation is performed by intimately contacting as by tumbling the two materials, or combinations of various carrying agents with molybdenum disulphide, in any desired proportions, desirably in a tumbling barrel until satisfactory adherence of the molybdenum disulphide to the carrier agent or agents is accomplished and no free or unadhered molybdenum disulphide is present. The receptacle, within which this operation takes place, should preferably be lined with some satisfactory type of lining material such as a suitable grade of plastic material, cork, rubber, or other similar material. This prevents the parts from becoming marred due to impact with a hard surface and also eliminates the possibility of the molybdenum disulphide building up on the surface of the receptacle, which it may tend to do if an unsatisfactory surface for this type of operation is present. The material carrying the molybdenum disulphide should be a material softer than the machine parts to be later tumbled, in order to prevent marring, and be of such a size that it may freely enter and be removed from any surface irregularities and any through or blind holes. The molybdenum disulphide is used in the dry condition so that a dry coating, very thin and very uniform is applied.

After the carrying agent or agents are thoroughly charged, the machine parts are introduced into this mixture and tumbledfor the desired length of time, dependent upon their size and shape, to coat them completely with a thin, uniform, film of molybdenum disulphide. Tumbling causes the parts to rub against the charged carrying agent. The charged carrier should be present in a volume ratio of for example, at least two parts of charged carrying agent to one part of machine element, in order to prevent impact of the parts upon one another. In so doing, sufficient molybdenum disulphide is transferred from the agent to the elements and adheres thereto,

It has been found, for example, that in charging the carrying agent, a ratio, by volume, of about twenty four (24) parts of carrying agent to one (1) part molybdenum disulphide will produce a satisfactorily charged particle and leave no free or unattached molybdenum disulphide in the barrel. Tumbling the parts in this mixture for, for example, from five (5) to ten minutes is usually long enough to result in a satisfactory film of molybdenum disulphide on the machine elements.

The ratio of carrying agent to molybdenum disulphide can be varied over a wide range in order to produce a desired product but should never be such that free or unattached molybdenum disulphide is present. Time of application is also widely variable and depends upon the conditions found most desirable. Additional molybdenum disulphide can be added to the carrying agent from time to time to maintain the desired concentration.

The film, so produced upon the parts, provides a su perior lubricating medium for repressing parts in any range of pressing pressures.

The advantages obtained in using powdered molybdenum disulphide, with the outlined method, resides in the fact that the film produced on the parts is always dry and no liquid phase is required, the molybdenum disulphide does not liquify, and this material will withstand extremely high pressures and temperatures without break ing down and losing its lubricating qualities. Molybdenum disulphide adheres to the metal surfaces, will not rub completely off when handled, is easy to apply, is relatively inexpensive due to the small quantities required, and keeps the tools clean and free from galling and roughening due to seizure. Molybdenum disulphide, as a dry material and applied by this method, does not penetrate the parts so treated and thus it does not fill any pores present in the parts with extraneous material, and is unique in that it has extremely good lubricating qualities under high pressure and high temperature conditions.

Despite the fact that the molybdenum disulphide will not penetrate into the porous structure of the metal of the machine element, the fine, dry lubricant powder will coat all surfaces, for example, teeth of small gears, and will penetrate between such surfaces that are closely contiguous to give the desired uniform coating essential to the present invention.

In summary, the advantages We have found in using MoS as a lubricant in a repressing or recoining operation on a powdered metal compact are: reduction of repressing pressures, reduction of compact ejection pressures, production of higher density for a given repressing pressure, production of a better surface finish on the machine part, reduction of surface porosity of the machine part, cleanliness of dies and punches and freedom from galling, elimination of entrapped deleterious material, desirable high temperature and high pressure lubricating qualities, the lubricant will not rub off completely with handling, no tool modifications for use are required, more uniformity is obtained in parts production, the production capacity is increased, subsequent operations are not affected, a desirable reduction in the required lubricating film depth, and provision of a measure of corrosion resistance to the parts. The advantages to be derived from the method include: simplicity of operation, rapidity of processing, economy of processing, elimination of entrapped liquids and solids, obtention of a uniform lubricating film on the compact, and the fact that no specialized equipment is required for practicing the method.

While I have thus described particular embodiments of my invention, it will be obvious to .those skilled in the art that various modifications and changes in proportions, temperatures, pressures and tumbling speeds may be made without departing from the spirit and scope of my invention.

What I claim is:

1. The method of preparing pre-formed powdered metal compacts for further pressure compacting comprising intimately contacting the compacts with finely divided dry powdered M08 in the presence of solid dry adsorbent material for the M08 said adsorbent material being softer than the compacts, and continuing such contacting operation for a time sufiicient to completely surface coat said compacts with M08 2. The method of claim 1 wherein the M08 is first coated on the adsorbent material which is selected from the group consisting of wire lengths, corn cobs, wood, leather, cork and rubber in physically reduced condition; and thereafter the coated adsorbent material is tumbled with the compacts.

3. The method of claim 2 wherein the ratio by volume of MoS coated adsorbent material to metal compacts is at least 2:1.

4. The method of claim 2 wherein the ratio by volume of MOS; to adsorbent material is about 1:24.

References Cited in the file of this patent UNITED STATES PATENTS Johnston May 10, 1904 Hardman Nov. 16, 1909 

1. THE METHOD OF PREPARING PRE-FORMED POWDERED METAL COMPACTS FOR FURTHER PRESSURE COMPACTING COMPRISING INTIMATELY CONTACTING THE COMPACTS WITH FINELY DIVIDED DRY POWDERED MOS2 IN THE PRESENCE OF SOLID DRY ADSORBENT MATERIAL FOR THE MOS2, SAID ADSORBENT MATERIAL BEING SOFTER THAN THE COMPACTS, AND CONTINUING SUCH CONTACTING OPERTION FOR A TIME SUFFICIENT TO COMPLETELY SURFACE COAT SAID COMPACTS WITH MOS2. 